FIG. 1 depicts a schematic diagram of wireless local-area network 100 in the prior art, which comprises: access point 101, stations 102-1 through 102-N, wherein N is a positive integer, and hosts 103-1 through 103-N, interconnected as shown. Each station 102-i, wherein i is a positive integer in the set {1, . . . N}, enables host 103-i (a device such as a notebook computer, personal digital assistant [PDA], tablet PC, etc.) to communicate wirelessly with other hosts in local-area network 100 via access point 101.
Access point 101 and stations 102-1 through 102-N transmit blocks of data called frames. A frame typically comprises a data portion, referred to as a data payload, and a control portion, referred to as a header. Frames transmitted from a station 102-i to access point 101 are referred to as uplink frames, and frames transmitted from access point 101 to a station 102-i are referred to as downlink frames. A series of frames transmitted from a station 102-i to access point 101 is referred to as an uplink traffic stream, and a series of frames transmitted from access point 101 to a station 102-i is referred to as a downlink traffic stream.
Access point 101 and stations 102-1 through 102-N transmit frames over a shared-communications channel such that if two or more stations (or an access point and a station) transmit frames simultaneously, then one or more of the frames can become corrupted (resulting in a collision). Consequently, local-area networks typically employ protocols for ensuring that a station or access point can gain exclusive access to the shared-communications channel for an interval of time in order to transmit one or more frames.
Such protocols can be classified into two types: contention-based protocols, and contention-free protocols. In a contention-based protocol, stations 102-1 through 102-N and access point 101 compete to gain exclusive access to the shared-communications channel, just as, for example, several children might fight to grab a telephone to make a call.
In a contention-free protocol, in contrast, a coordinator (e.g., access point 101, etc.) grants access to the shared-communications channel to one station at a time. An analogy for contention-free protocols is a parent (i.e., the coordinator) granting each of several children a limited amount of time on the telephone to talk, one at a time. One technique in which a coordinator can grant access to the shared-communications channel is polling. In protocols that employ polling, stations submit a polling request (also referred to as a reservation request) to the coordinator, and the coordinator grants stations exclusive access to the shared-communications channel sequentially in accordance with a polling schedule. A polling schedule has a temporal period (e.g., 5 seconds, etc.) and continually loops back to the beginning of the schedule after its completion. Since stations transmit only in response to a poll from the coordinator, polling-based protocols can provide contention-free access to the shared-communications channel.